This invention relates to a storage stable lubricating composition containing an additive package which helps to provide particularly improved anti-friction and anti-wear properties.
There are many instances, as is well known, particularly under "Boundary Lubrication" conditions where two rubbing surfaces must be lubricated, or otherwise protected, so as to prevent wear and to insure continued movement. Moreover, where, as in most cases, friction between the two surfaces will increase the power required to effect movement and where the movement is an integral part of an energy conversion system, it is most desirable to effect the lubrication in a manner which will minimize this friction. As is also well known, both wear and friction can be reduced, with various degrees of success, through the addition of a suitable additive or combination thereof, to a natural or synthetic lubricant. Similarly, continued movement can be insured, again with varying degrees of success, through the addition of one or more appropriate additives.
While there are many known additives which may be classified as anti-wear, anti-friction and extreme pressure agents and some may in fact satisfy more than one of these functions as well as provide other useful functions, it is also known that many of these additives act in a different physical or chemical manner and often compete with one another, e.g. they may compete for the surface of the moving metal parts which are subjected to lubrication. Accordingly, extreme care must be exercised in the selection of these additives to insure compatibility and effectiveness.
The metal dihydrocarbyl dithiophosphates are one of the additives which are known to exhibit antioxidant and anti-wear properties. The most commonly used additives of this class are the zinc dialkyl dithiophosphates which are conventionally used in lubricant compositions. While such zinc compounds afford excellent oxidation resistance and exhibit superior anti-wear properties, it has heretofore been believed that the same increases or significantly limits the ability to decrease friction between moving surfaces. As a result, compositions containing zinc dialkyl dithiophosphates were not believed to provide the most desirable lubricity and, in turn, it was believed that use of compositions containing the same would lead to significant energy losses in overcoming friction even when anti-friction agents are included in the composition.
Known ways to solve the problem of energy losses due to high friction, e.g. in crankcase motor oils include the use of synthetic ester base oils which are expensive and the use of insoluble molybdenum sulfides which have the disadvantage of giving the oil composition a black or hazy appearance.
Additive mixtures of oil soluble dimer acids and polyols as disclosed in U.S. Pat. No. 3,180,832 and the esters prepared by the reaction of such components as disclosed in U.S. Pat. No. 3,429,817 exhibit good anti-wear properties as reported in said patents. The mixtures as shown in the 3,180,832 patent were also shown to have friction reducing properties. However, the use of such additives did not appear to present a practical alternative for use in conventional oils containing zinc dialkyl dithiophosphates for lubrication under boundary conditions (e.g. crankcase oils) where the prevention of wear due to heavy loading is a serious problem and the zinc dialkyl dithiophosphate is used because of its anti-wear as well as extreme pressure properties. This was based on the fact that the mixtures as taught in U.S. Pat. No. 3,180,832 were not useful in crankcase motor oils since the acid component is corrosive and interacts with the conventional zinc compound generally used for minimizing valve lifter wear and if the lower cost short chain glycols were used in order to make the mixture more commercially feasible, these shorter chain glycols would boil off under normal use conditions. Furthermore, the ester compounds as taught in U.S. Pat. No. 3,429,817 also tend to interact with the zinc dialkyl dithiophosphate and can cause such additives to eventually precipitate or drop out of the lubricant composition, i.e. it is an unstable composition.
In light of the foregoing, the need for improved lubricating compositions that will permit operation of moving parts under boundary conditions with reduced friction is believed to be readily apparent. Similarly, the need for such a composition that can include conventional base oils and other conventional additives and can be used without the loss of other desirable lubricant properties, particularly those provided by zinc dialkyl dithiophosphates, is also readily apparent.